1. Field of the Invention
This invention relates to a method and apparatus for the manufacture of concrete products. More particularly, this invention is concerned with the manufacture of terrazzo panels and the like.
2. Description of the Prior Art
Concrete as it is employed in this specification refers to the stone-like product obtained by blending a cement, aggregates such as sand, gravel, crushed stone and the like, and water together in appropriate amounts, and allowing the mixture to harden and cure. Terrazzo as used in this specification refers to a special form of concrete in which the aggregate used is selected so as to impar a decorative effect to the final product when exposed by grinding and polishing. The aggregate used in terrazzo can include, for example, marble dust, marble chips, granite chips, colored stones of various types, broken glass and the like.
In the prior art methods of manufacturing concrete products, the cement, aggregate, additives if any, and water are blended in various ratios. In general, a sufficient amount of water is added to completely hydrate the cement and also impart a degree of fluidity to the blended mixture. The blended mixture is then poured or compacted into a form and allowed to take an initial set. The initial set usually occurs within one-half hour to four hours, depending upon the temperature and the particular concrete mixture employed. The concrete product, after this initial set, is solid, but has very little strength. In order to obtain the maximum strength possible with a given concrete composition using the prior art manufacturing techniques, the concrete is generally allowed to cure for a period of time from several days to several weeks.
The excessively long period of time required for both the initial set, and particularly for the development of the ultimate strength, causes considerable problems in the manufacture of cast concrete products. The concrete mixture must remain in the mold until the product acquires sufficient strength that it will not fall apart when the mold is removed. Furthermore, the concrete product, even after the mold is removed, must be allowed to cure for an extended period of time before it is subjected to any substantial stresses. The need to maintain the concrete products in the mold until the initial set is obtained, and for curing the products until the ultimate strength is developed requires a substantial inventory of molds and a rather large storage area for the products being processed. Of even greater consequence is the cost involved in the prior art processes since the handling of the weak products obtained after the initial set and the storage of the product for several days or weeks until fully cured is quite costly in both manpower and storage space.
The problems encountered in handling and storage of concrete products using prior art processes can best be illustrated by the conventional methods of manufacturing terrazzo panels. In the conventional method a concrete mixture containing the desired type of aggregate is either poured or compacted into a suitable panel mold. The mold is then stored until the mixture initially sets. At this point the panel cannot be ground or polished since the machining would cause the panel to disintegrate, especially if the grinding were conducted at the high speeds required for commercial production of terrazzo panels. Accordingly, the panel has to be stored for days or weeks until fully cured, and then finished.
A further problem encountered with cast concrete products in general, and terrazzo products in particular, is that often, using the prior art processes, products are obtained which lack sufficient compressive strength. In order to compensate for the relatively low compressive strength, the products have to be cast with thicker cross sections so as to provide the required strength. This materially adds to the cost of the product, the cost of transportation of the finished product, and the cost of installing the product. As can be seen most readily with terrazzo panels which are used as floorings in many public buildings and the like, if the panels can be made stronger, thinner sections can be used as these would have the same compressive strength as the weaker, thicker panels. The use of the thinner terrazzo floor panels directly reduces the material cost of the flooring, and indirectly would reduce the total cost of the building, in that the supporting structures for the flooring would not have to be as strong in order to support the lighter weight terrazzo panels.
Attempts have heretofore been made to decrease the cure time in the manufacture of concrete products. These included adding various types of accelerators to the concrete mixtures as well as heating or steaming the concrete products during manufacture. These methods at best produced marginal improvements with regard to reducing the cure time, and were somewhat expensive. For example, in high pressure steam curing the product often had to be steamed for 12 to 24 hours in order to obtain a satisfactory hardness and strength in the final product. In addition, certain of the processes had adverse effects on the composition. High pressure steaming of the concrete products containing substantial amounts of marble or limestone, for example, caused a very marked decrease in the ultimate compressive strength of the final product. Accordingly, these processes could not be used with terrazzo products which usually contain substantial amounts of marble dust or marble chips.
In order to increase the strength of concrete products certain additives have heretofore been suggested. These additives include materials which to some extent plasticize the finished concrete product so at to increase its compressive strength. Additives have been suggested to improve the properties of concrete products, including bituminous materials and various types of rubbers and emulsions of synthetic resins. One patent which has been found to be particularly interesting because it is concerned primarily with terrazzo-type concrete products is Murray, U.S. Pat. No. 3,305,506. Murray teaches the use of a vinyl acetate-dibutyl maleate copolymer for improving the compressive strength. It should be noted, however, that while these additives do have a definite beneficial effect on improving the compressive strength, the ultimate compressive strength is still relatively low. This limits the use of concrete products having thin cross sections in applications which require the the material employed to carry considerable compressive loads.
It is accordingly an object of this invention to overcome the aforementioned problems and difficulties of the prior art.
It is the more specific object of this invention to provide a method and apparatus for rapidly curing concrete products.
It is an additional object of this invention to provide a process for producing concrete products with improved strength.
It is a still more specific object of this invention to provide a method and apparatus for continuously manufacturing terrazzo-type concrete products.
Other objects and advantages of this invention will become further apparent from a study of the attached figures and a continued reading of the specification and subjoined claims.